Summary

Obsolete the no longer used flag UseNeon and also schedule its expiration.

Problem

The flag UseNeon has no effect as of JDK-8328264 in JDK 23.

Solution

Obsolete UseNeon in JDK 23 and expire it in JDK 24.

Specification

diff --git a/src/hotspot/cpu/aarch64/globals_aarch64.hpp b/src/hotspot/cpu/aarch64/globals_aarch64.hpp
index 760aada0270..2f83838fc0f 100644
--- a/src/hotspot/cpu/aarch64/globals_aarch64.hpp
+++ b/src/hotspot/cpu/aarch64/globals_aarch64.hpp
@@ -85,8 +85,6 @@ define_pd_global(intx, InlineSmallCode, 1000);
                                                                         \
   product(bool, NearCpool, true, \
          "constant pool is close to instructions") \
- product(bool, UseNeon, false, \
- "Use Neon for CRC32 computation") \
   product(bool, UseCRC32, false, \
           "Use CRC32 instructions for CRC32 computation") \
   product(bool, UseCryptoPmullForCRC32, false, \
diff --git a/src/hotspot/share/runtime/arguments.cpp b/src/hotspot/share/runtime/arguments.cpp
index ecd3f00cbeb..8ccd466d109 100644
--- a/src/hotspot/share/runtime/arguments.cpp
+++ b/src/hotspot/share/runtime/arguments.cpp
@@ -537,6 +537,7 @@ static SpecialFlag const special_jvm_flags[] = {

   { "ParallelOldDeadWoodLimiterMean", JDK_Version::undefined(), JDK_Version::jdk(23), JDK_Version::jdk(24) },
   { "ParallelOldDeadWoodLimiterStdDev", JDK_Version::undefined(), JDK_Version::jdk(23), JDK_Version::jdk(24) },
+ { "UseNeon", JDK_Version::undefined(), JDK_Version::jdk(23), JDK_Version::jdk(24) },
 #ifdef ASSERT
   { "DummyObsoleteTestFlag", JDK_Version::undefined(), JDK_Version::jdk(18), JDK_Version::undefined() },
 #endif
diff --git a/src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/hotspot/aarch64/AArch64HotSpotJVMCIBackendFactory.java b/src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/hotspot/aarch64/AArch64HotSpotJVMCIBackendFactory.java
index 9d7d4748fdd..890a3357678 100644
--- a/src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/hotspot/aarch64/AArch64HotSpotJVMCIBackendFactory.java
+++ b/src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/hotspot/aarch64/AArch64HotSpotJVMCIBackendFactory.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2024, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -58,9 +58,6 @@ private static EnumSet<AArch64.Flag> computeFlags(AArch64HotSpotVMConfig config)
         if (config.useCRC32) {
             flags.add(AArch64.Flag.UseCRC32);
         }
- if (config.useNeon) {
- flags.add(AArch64.Flag.UseNeon);
- }
         if (config.useSIMDForMemoryOps) {
             flags.add(AArch64.Flag.UseSIMDForMemoryOps);
         }
diff --git a/src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/hotspot/aarch64/AArch64HotSpotVMConfig.java b/src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/hotspot/aarch64/AArch64HotSpotVMConfig.java
index 720bebb30cf..a388687c69a 100644
--- a/src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/hotspot/aarch64/AArch64HotSpotVMConfig.java
+++ b/src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/hotspot/aarch64/AArch64HotSpotVMConfig.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016, 2022, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2016, 2024, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -44,7 +44,6 @@ class AArch64HotSpotVMConfig extends HotSpotVMConfigAccess {
      * These flags are set based on the corresponding command line flags.
      */
     final boolean useCRC32 = getFlag("UseCRC32", Boolean.class);
- final boolean useNeon = getFlag("UseNeon", Boolean.class);
     final boolean useSIMDForMemoryOps = getFlag("UseSIMDForMemoryOps", Boolean.class);
     final boolean avoidUnalignedAccesses = getFlag("AvoidUnalignedAccesses", Boolean.class);
     final boolean useLSE = getFlag("UseLSE", Boolean.class);

Full PR for reference: https://github.com/openjdk/jdk/pull/18648